1. Field of Invention
The present invention relates generally to wind extraction apparatus, and more particularly to wind extraction apparatus utilizing a ducted wind turbine with the rotor positioned aft of a converging section of the duct and incorporating a unique rotor design which accounts for the flow field created by the duct.
2. Background of Art
The proposed invention pertains to the increasing need for renewable, sustainable and green energy sources. Wind energy has long been acknowledged as having vast potential to fulfill these needs. The wide adoption of small wind energy has been hampered by higher unit costs and lower efficiency and the described concept seeks to alleviate this difference.
Ducted wind turbines (DWTs) are created by enclosing a conventional horizontal axis wind turbine (HAWT) with a duct, which can be an aerodynamic curved surface or ideally, an airfoil revolved around the rotor axis. The presence of the duct increases the mass flow rate and velocity through the turbine. In a paper by Foreman, K. M., Gilbert, B. L., and Oman, R. A., “Diffuser Augmentation of Wind Turbines”, Journal of Solar Energy, Vol. 20, No. 4, April 1978, pp. 305-311, these ducted turbines were extensively tested in the 1970's and 1980's, and it was proposed that this improvement in performance occurred because the duct reduces the pressure behind the turbine, relative to that behind a conventional wind turbine, causing more air to be drawn through. Hanson proposed in 2008 that it is the lift generated by the shroud, as shown by de Vries in 1979 that induces an increased mass flow through the rotor, resulting in an increase in the power coefficient proportional to the mass flow. For either position, the presence of the duct increases the mass flow rate and, consequently, the power output of the turbine. The duct captures a much larger stream tube than an open rotor configuration and a substantial increase in velocity, exceeding even the free stream, is observed at the rotor face.
The theoretical maximum open rotor power extractable from the wind, based on a streamtube the diameter of the rotor, is 59.3% and is known as the Betz limit. The power coefficient is thus defined as:Cp=power extracted/power in the wind=2P/ρπR2V3 where Cpmax=CpBetz=0.593, and R=rotor radius. With an increased mass flow rate and velocity, a DWT increases the amount of generated power and Cp values can exceed the Betz limit when based on the rotor area R2. Gilbert and Foreman suggested that they could have a Cp of 1.57, leading to an ‘augmentation ratio’ of CpDWT/CpBetz=2.65. Many studies have investigated the feasibility and associated augmentation factors seen in DWTs [Hu et al, 2008; Igra O., 1976, 1984; Hansen et al, 2000; Werle and Presz 2008; Van Bussel, 2007; Oman, 1977; Leoffler, A. L. and Vanderbilt, 1978; Riegler, 1983; Politis and Koras, 1995] with the largest prediction of 7 by Badawy and Aly [2000], however conclusions have been quite varied. Werle and Presz [2008] used fundamental momentum principles and concluded that the possible augmentation factor could only approach 2, and that earlier studies had incorrect assumptions, leading to overly optimistic predictions. Hansen [2000] has published viscous CFD results that predicted ideal Cp values approaching 0.94, and an augmentation factor of 1.6. He also indicated that if the duct geometry could be made to keep the flow attached, the augmentation factor could be improved further. This potential increase in power generation has driven research into DWTs, however to date, no scaled-up experimental design has been able to realize these augmentation factors and no commercially viable DWT has been successful. A good example of this type of failure is seen in the Vortec 7 from New Zealand. [Phillips et al, 2003].
DWTs have been proposed to offer additional advantages to the increased mass flow augmentation, such as minimizing tip losses and being less yaw sensitive to ambient winds than HAWTs. Studies of DWTs at Clarkson University, [Moeller and Visser, 2010; Venters, 2014] have indicated Cp values, based on the rotor area, above 1.1 and additional design benefits such as acoustic signature reduction and no tail requirements. Perhaps, best of all, is that the overall potential for higher energy extraction at lower speeds opens up many more areas of the country to a viable distributed wind energy solution.
In order to extract this increased power, a wind turbine rotor is required in the same manner as an open rotor. Most rotor designs seek to exploit the high velocity at the duct, however the presence of the rotor itself actually modifies the velocity where it is stationed and the duct itself modifies the velocity field the rotor sees such that the optimum blade design for the rotor is not that which would be required of an open rotor. In fact, it is quite different in planform shape and in twist, due to the presence of the flowfield generated by the duct.
An example of an arrangement to exploit the use of a duct in such a manner is described in U.S. Pat. No. 7,018,166 where a rotor is placed at the throat of the duct, wherein the velocity is the highest. In this description, an additional second rotor is placed downstream of the duct, termed the free rotor, but is driven by the flow external to the duct, not the internal flow field. This and other similar descriptions seek to exploit the high velocity at the duct throat, however the optimum arrangement for maximum power extraction of a given ducted turbine is a rotor positioned downstream of the throat, as will be noted in the present description.